Ramban #1 (citing R. Shmuel ben Chofni): The challenge ¹ was based on the assumption that Pharaoh would think that astrologically speaking, now was the time for the plague to go away, so he would say tomorrow. ²

Ramban #2: Because Pharaoh thought that Moshe's request implied a short time-period - and did not incorporate 'today.'

Moshav Zekenim #1 (to 8:1): He thought that naturally, frogs can live up to a day outside the water, so he assumed that, by the following day they would all be dead, proving that Moshe was an imposter.

Moshav Zekenim #2 (to 8:5): Pharaoh claimed to be a god. Consequently, he did not want to reveal that the plague pained him.

Riva: Pharaoh was responding that he will send out Yisrael tomorrow. Refer to 8:5:2:5.

Gur Aryeh (to 8:5): Pharaoh knew that the Makah could not be removed on a day when it was already in force; conceptually, the day itself had already been smitten. Although Arov ³ and Barad would stop immediately upon Moshe's Tefilah, that is because it is natural for such events to come along and then move on. But the frogs would have to suddenly drop dead - and a salvation of that degree would not happen on the same day. ⁴

Ramban: Refer to 8:5:3:1.

Maharal: Refer to 7:26:152:1, and 7:26:152:1 ¹ .